Buffered output typically comes in two flavors: fully buffered and line buffered. If output goes to a terminal then the implementation automatically selects line buffering, flushing its buffer when a line is completed so that it reaches the user immediately. Otherwise it fully buffers, holding output until the buffer…
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In software development there are many concepts that at first glance seem useful and sound, but, after considering the consequences of their implementation and use, are actually horrifying. Examples include thread cancellation , variable length arrays , and memory aliasing . GCC’s closure extension to C is another…
Over the past year I’ve refined my approach to arena allocation . With practice, it’s effective, simple, and fast; typically as easy to use as garbage collection but without the costs. Depending on need, an allocator can weigh just 7–25 lines of code — perfect when lacking a runtime . With the core details of my own…
For some time Elfeed was experiencing a strange, spurious failure. Every so often users were seeing an error (spoiler warning) when updating feeds: “error in process sentinel: Search failed.” If you use Elfeed, you might have even seen this yourself. From the surface it appeared that curl, tasked with the…
I’ve noticed a small pattern across a few of my projects where I had vectorized and parallelized some code. The original algorithm had a “push” approach, the optimized version instead took a “pull” approach. In this article I’ll describe what I mean, though it’s mostly just so I can show off some pretty videos…
Linux has an elegant and beautiful design when it comes to threads: threads are nothing more than processes that share a virtual address space and file descriptor table. Threads spawned by a process are additional child processes of the main “thread’s” parent process. They’re manipulated through the same process…
Today there was a question on /r/C_Programming about the effect of C’s const on optimization. Variations of this question have been asked many times over the past two decades. Personally, I blame naming of const .
Now they’ve gone an done it. An unidentified agency has spread a potent computer virus across all the world’s computers and deleted the binaries for every copy of every software development tool. Even the offline copies — it’s that potent.
While considering concurrent queue design I came up with a generic, lock-free queue that fits in a 32-bit integer. The queue is “generic” in that a single implementation supports elements of any arbitrary type, despite an implementation in C. It’s lock-free in that there is guaranteed system-wide progress. It can…
I recently learned of csvquote , a tool that encodes troublesome CSV characters such that unix tools can correctly process them. It reverses the encoding at the end of the pipeline, recovering the original input. The original implementation handles CSV quotes using the straightforward, naive method. However, there’s a…
In this post I’m going to do a silly, but interesting, exercise that should never be done in any program that actually matters. I’m going write a program that changes one of its function definitions while it’s actively running and using that function. Unlike last time , this won’t involve shared libraries, but it will…
Occasionally I’ve needed to key a hash table with C pointers. I don’t care about the contents of the object itself — especially if it might change — just its pointer identity. For example, suppose I’m using null-terminated strings as keys and I know these strings will always be interned in a common table. These…
Windows paths have been WTF-16 -encoded for decades, but module names in the import tables of Portable Executable are octets. If a name contains values beyond ASCII — technically out of spec — then the dynamic linker must somehow decode those octets into Unicode in order to construct a lookup path. There are multiple…
The Windows API — a.k.a. Win32 — is notorious for being clunky, ugly, and lacking good taste. Microsoft has done a pretty commendable job with backwards compatibility, but the trade-off is that the API is filled to the brim with historical cruft. Every hasty, poor design over the decades is carried forward forever…
I’m a huge fan of interactive programming (see: JavaScript , Java , Lisp , Clojure ). That is, modifying and extending a program while it’s running. For certain kinds of non-batch applications, it takes much of the tedium out of testing and tweaking during development. Until last week I didn’t know how to apply…
Suppose you’re not using a C runtime on Linux, and instead you’re programming against its system call API. It’s long-term and stable after all. Memory management and buffered I/O are easily solved, but a lot of software benefits from concurrency. It would be nice to also have thread spawning capability. This article…
In preparation for a future project, I was thinking about at the unix find utility . It operates a file system hierarchies, with basic operations selected and filtered using a specialized expression language. Users compose operations using unary and binary operators, grouping with parentheses for precedence. find may…
Monday’s /r/dailyprogrammer challenge was to write a program to read a recurrence relation definition and, through interpretation, iterate it to some number of terms. It’s given an initial term ( u(0) ) and a sequence of operations, f , to apply to the previous term ( u(n + 1) = f(u(n)) ) to compute the next term…
Recently I’ve been experimenting with freestanding C programs on Windows. Freestanding refers to programs that don’t link, either statically or dynamically, against a standard library (i.e. libc). This is typical for operating systems and similar, bare metal situations . Normally a C compiler can make assumptions…
Suppose you have multiple processes appending output to the same file without explicit synchronization. These processes might be working in parallel on different parts of the same problem, or these might be threads blocked individually reading different external inputs. There are two concerns that come into play:
A common situation in asyncio Python programs is asynchronous initialization. Some resource must be initialized exactly once before it can be used, but the initialization itself is asynchronous — such as an asyncpg database. Let’s talk about a couple of solutions.
Back in 2017 I wrote about a technique for creating closures in C using JIT-compiled wrapper. It’s neat, though rarely necessary in real programs, so I don’t think about it often. I applied it to qsort , which sadly accepts no context pointer. More practical would be working around insufficient custom allocator…